A spark plug generates a spark at a gap formed between a distal end portion of a center electrode and a distal end portion of a ground electrode upon application of a voltage between the center electrode and the ground electrode, which are insulated from each other by means of an insulator. For example, Japanese Patent Application Laid-Open (kokai) No. 2002-237365 discloses a spark plug in which the center electrode and the ground electrode face each other in the axial direction so as to form a gap.
Incidentally, in order to decrease the fuel cost of an internal combustion engine and/or purify exhaust gas, the degree of leanness of air-fuel mixture and/or the amount of re-circulated gas (EGR gas) has been increased. In order to compensate for a decrease in flame propagation speed caused by the increased degree of leanness of air-fuel mixture and/or the increased amount of re-circulated gas, the speed of a gas flow within a combustion chamber of the internal combustion engine tends to be increased. As a result, in the spark plug of Patent Document 1, a phenomenon in which the spark generated at the gap of the spark plug is blown by the gas flow and is quenched (spark blowout) becomes likely to occur. The spark blowout prevents extension of a spark discharge path to thereby deteriorate the ignition performance of the spark plug, and causes multiple discharge to thereby deteriorate the durability of the spark plug. Therefore, there has been a demand for a technique of securing the required ignition performance and durability of the spark plug even when the speed of a gas flow within a combustion chamber is high.
An object of the present invention is to secure the required ignition performance and durability of a spark plug even when the speed of a gas flow within a combustion chamber is high.